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--[[

   

--]]

---
-- a flexible statement based load balancer with connection pooling
--
-- * build a connection pool of min_idle_connections for each backend and 
--   maintain its size
-- * reusing a server-side connection when it is idling
-- 

--- config
--
-- connection pool
local min_idle_connections = 4
local max_idle_connections = 8

-- debug
local is_debug = true

--- end of config

---
-- read/write splitting sends all non-transactional SELECTs to the slaves
--
-- is_in_transaction tracks the state of the transactions
local is_in_transaction = 0

--- 
-- get a connection to a backend
--
-- as long as we don't have enough connections in the pool, create new connections
--
function connect_server() 
    -- make sure that we connect to each backend at least ones to 
    -- keep the connections to the servers alive
    --
    -- on read_query we can switch the backends again to another backend

    if is_debug then
        print()
        print("[connect_server] ")
    end

    local least_idle_conns_ndx = 0
    local least_idle_conns = 0

    for i = 1, #proxy.global.backends do
        local s = proxy.global.backends[i]
        local pool     = s.pool -- we don't have a username yet, try to find a connections which is idling
        local cur_idle = pool.users[""].cur_idle_connections

        if is_debug then
            print("  [".. i .."].connected_clients = " .. s.connected_clients)
            print("  [".. i .."].idling_connections = " .. cur_idle)
            print("  [".. i .."].type = " .. s.type)
            print("  [".. i .."].state = " .. s.state)
        end

        if s.state ~= proxy.BACKEND_STATE_DOWN then
            -- try to connect to each backend once at least
            if cur_idle == 0 then
                proxy.connection.backend_ndx = i
                if is_debug then
                    print("  [".. i .."] open new connection")
                end
                return
            end

            -- try to open at least min_idle_connections
            if least_idle_conns_ndx == 0 or
               ( cur_idle < min_idle_connections and 
                 cur_idle < least_idle_conns ) then
                least_idle_conns_ndx = i
                least_idle_conns = s.idling_connections
            end
        end
    end

    if least_idle_conns_ndx > 0 then
        proxy.connection.backend_ndx = least_idle_conns_ndx
    end

    if proxy.connection.backend_ndx > 0 then 
        local s = proxy.global.backends[proxy.connection.backend_ndx]
        local pool     = s.pool -- we don't have a username yet, try to find a connections which is idling
        local cur_idle = pool.users[""].cur_idle_connections

        if cur_idle >= min_idle_connections then
            -- we have 4 idling connections in the pool, that's good enough
            if is_debug then
                print("  using pooled connection from: " .. proxy.connection.backend_ndx)
            end
    
            return proxy.PROXY_IGNORE_RESULT
        end
    end

    if is_debug then
        print("  opening new connection on: " .. proxy.connection.backend_ndx)
    end

    -- open a new connection 
end

--- 
-- put the successfully authed connection into the connection pool
--
-- @param auth the context information for the auth
--
-- auth.packet is the packet
function read_auth_result( auth )
    if auth.packet:byte() == proxy.MYSQLD_PACKET_OK then
        -- auth was fine, disconnect from the server
        proxy.connection.backend_ndx = 0
    elseif auth.packet:byte() == proxy.MYSQLD_PACKET_EOF then
        -- we received either a 
        -- 
        -- * MYSQLD_PACKET_ERR and the auth failed or
        -- * MYSQLD_PACKET_EOF which means a OLD PASSWORD (4.0) was sent
        print("(read_auth_result) ... not ok yet");
    elseif auth.packet:byte() == proxy.MYSQLD_PACKET_ERR then
        -- auth failed
    end
end


--- 
-- read/write splitting
function read_query( packet ) 
    if is_debug then
        print("[read_query]")
        print("  authed backend = " .. proxy.connection.backend_ndx)
        print("  used db = " .. proxy.connection.client.default_db)
    end

    if packet:byte() == proxy.COM_QUIT then
        -- don't send COM_QUIT to the backend. We manage the connection
        -- in all aspects.
        proxy.response = {
            type = proxy.MYSQLD_PACKET_OK,
        }

        return proxy.PROXY_SEND_RESULT
    end

    if proxy.connection.backend_ndx == 0 then
        -- we don't have a backend right now
        -- 
        -- let's pick a master as a good default
        for i = 1, #proxy.global.backends do
            local s = proxy.global.backends[i]
            local pool     = s.pool -- we don't have a username yet, try to find a connections which is idling
            local cur_idle = pool.users[proxy.connection.client.username].cur_idle_connections
            
            if cur_idle > 0 and 
               s.state ~= proxy.BACKEND_STATE_DOWN and 
               s.type == proxy.BACKEND_TYPE_RW then
                proxy.connection.backend_ndx = i
                break
            end
        end
    end

    if true or proxy.connection.client.default_db and proxy.connection.client.default_db ~= proxy.connection.server.default_db then
        -- sync the client-side default_db with the server-side default_db
        proxy.queries:append(2, string.char(proxy.COM_INIT_DB) .. proxy.connection.client.default_db,  { resultset_is_needed = true })
    end
    proxy.queries:append(1, packet,  { resultset_is_needed = true })

    return proxy.PROXY_SEND_QUERY
end

---
-- as long as we are in a transaction keep the connection
-- otherwise release it so another client can use it
function read_query_result( inj ) 
    local res      = assert(inj.resultset)
    local flags    = res.flags

    if inj.id ~= 1 then
        -- ignore the result of the USE <default_db>
        return proxy.PROXY_IGNORE_RESULT
    end
    is_in_transaction = flags.in_trans

    if not is_in_transaction then
        -- release the backend
        proxy.connection.backend_ndx = 0
    end
end

--- 
-- close the connections if we have enough connections in the pool
--
-- @return nil - close connection 
--         IGNORE_RESULT - store connection in the pool
function disconnect_client()
    if is_debug then
        print("[disconnect_client]")
    end

    if proxy.connection.backend_ndx == 0 then
        -- currently we don't have a server backend assigned
        --
        -- pick a server which has too many idling connections and close one
        for i = 1, #proxy.global.backends do
            local s = proxy.global.backends[i]
            local pool     = s.pool -- we don't have a username yet, try to find a connections which is idling
            local cur_idle = pool.users[proxy.connection.client.username].cur_idle_connections

            if s.state ~= proxy.BACKEND_STATE_DOWN and
               cur_idle > max_idle_connections then
                -- try to disconnect a backend
                proxy.connection.backend_ndx = i
                if is_debug then
                    print("  [".. i .."] closing connection, idling: " .. cur_idle)
                end
                return
            end
        end
    end
end
